1. Field of the Invention: The invention relates to a flame detecting thermocouple which can detect an established flame and deliver an electric signal. The flame detecting thermocouple makes use of two dissimilar metallic elements which have their ends welded together and the elements are connected to conductors properly insulated to deliver an electric signal as the voltage is developed across the conductors to control the operation of a gas valve such as disclosed in the Paul Dietiker U.S. Pat. No. 3,877,475 issued Apr. 15, 1975.
2. Description of the Prior Art: Flame detecting thermocouples are broadly old and have been used to prove the pilot flame of gas burners for many years. As the thermocouples are used in gas furnace control systems, a number of rigid requirements must be met. Obviously, the thermoelectromotive force or voltage developed must be sufficient to maintain the control device in an energized position when there is the presence of a flame. The response time to develop the voltage upon an initial startup must be sufficiently low to insure that the voltage is developed without requiring a manual actuator to be held in for too long a period of time. The heat resistance properties of the thermocouple must be sufficient to withstand many hours at a high temperature. The anti-corrosive properties of the thermocouple which operates in a high temperature oxidizing atmosphere containing gases and sulfur must be sufficient to provide a thermocouple which operates under these conditions for a considerable period of time. The thermocouple must be easily manufactured to maintain a good workable thermocouple at a low cost so the manufacturer can produce the thermocouple and yet market it to the ultimate consumer at a reasonable price in a very competitive market.
Many prior art thermocouples use an iron alloy for one element and a copper alloy such as Constantan or Copel as a second element and the elements are welded together to provide the electromotive output.